This study is designed to determine the number of loci within the rabbit major histocompatibility complex (MHC) and their chromosomal arrangements. We have identified the locus controlling the mixed leukocyte response and are developing estimates of the amount of polymorphism at this locus by the use of homozygous typing cells. We are preparing monospecific typing antisera for the serologically detected loci within the MHC and hope to provide typing antisera for animals from outside our present colony.